1. Field of the Invention
The present invention relates to a magnetic recording medium where a servo pattern is formed by a concave/convex pattern in a servo pattern region, a recording/reproducing apparatus equipped with the magnetic recording medium, and a stamper for manufacturing the magnetic recording medium.
2. Description of the Related Art
The specification of U.S. Pat. No. 5,772,905 discloses a nanoimprint lithography method (an imprint method that forms a concave/convex pattern of nanometer size: hereinafter simply “imprint method”) that forms a concave/convex pattern of nanometer size on a substrate by pressing a stamper on which a concave/convex pattern of nanometer size is formed into a resin layer on the substrate to transfer the concave/convex pattern of the stamper to the resin layer during a manufacturing process of a semiconductor element, an information recording medium, or the like. In this imprint method, first a stamper (“mold” in U.S. Pat. No. 5,772,905) with a concave/convex pattern of nanometer size (as one example, a minimum lateral size of around 25 nm) formed on a transfer surface thereof is manufactured. More specifically, a desired pattern is drawn on a resin layer formed so as to cover a thin film (“molding layer”) of silicon oxide or the like formed on the surface of a silicon substrate using an electron beam lithography device, and then a concave/convex pattern with a plurality of convex parts (“features”) is formed in the thickness of the thin film by etching the resin layer using a reactive ion etching device with the thin film as a mask. By doing so, the stamper is manufactured.
Next, as one example, the resin material polymethyl methacrylate (PMMA) is spin-coated onto the surface of a silicon substrate to form a resin layer (“thin film layer”) with a thickness of around 55 nm. After this, both the stamper and the multilayer body composed of the substrate and the resin layer are heated and the convex parts of the stamper are pressed into the resin layer on the substrate. When doing so, the resin material at positions where the convex parts of the stamper are pressed in moves into the concave parts of the stamper, resulting in concave parts (“regions”) being formed at (i.e., transferred to) the positions where the convex parts are pressed in. Next, after the multilayer body has been allowed to reach room temperature in a state where the stamper is still attached (i.e., after a cooling process has been carried out), the stamper is separated from the resin layer. By doing so, the convex parts in the concave/convex pattern of the stamper are transferred to the resin layer, thereby forming a concave/convex pattern of nanometer size (in the resin layer) on the substrate. After this, by etching the substrate using the resin layer in which the concave/convex pattern is formed as a mask, a plurality of concave parts are formed in the substrate. Accordingly, by forming concave parts in the resin layer on an information recording medium substrate using the technique (“imprint method”) described above, it is possible to manufacture an information recording medium with a concave/convex pattern of nanometer size by etching using the resin layer as a mask.
However, by investigating the information recording medium (magnetic recording medium) manufactured according to the conventional imprint method (manufacturing method) described above, the present inventors discovered the following problem. According to the conventional manufacturing method, a concave/convex pattern is formed by producing concave parts in a resin layer formed on a substrate by pressing convex parts of a stamper into the resin layer and then forming a concave/convex pattern in the substrate by etching the substrate using the resin layer in which the concave/convex pattern has been formed as a mask. However, when a discrete track magnetic recording medium is manufactured according to this manufacturing method, the convex parts (of the stamper) are insufficiently pressed into the resin layer. During a process that removes resin material remaining at base parts of the concave parts in the concave/convex pattern formed in (i.e., transferred to) the resin layer, the open surfaces of the concave parts become excessively wide, and therefore it can be difficult to form a concave/convex pattern on a magnetic recording medium with high precision.
A specific example is shown in FIG. 31. A magnetic disk 10z manufactured according to the method of manufacturing described above is manufactured by setting data recording regions Atz, in which data track patterns 40tz respectively composed of a plurality of concentric data recording tracks are formed, and servo pattern regions Asz, in which servo patterns 40sz for tracking servo purposes are formed, so as to alternate in the direction of rotation (the direction of the arrow R in FIG. 31) of the magnetic disk 10z. Here, as shown in FIG. 32, a servo pattern region Asz of the magnetic disk 10z includes for example a preamble pattern region Apz in which a preamble pattern is formed, an address pattern Aaz in which an address pattern is formed, and a burst pattern region Abz where burst patterns are formed in the burst regions Ab1z to Ab4z. Here, non-servo signal regions Axz constructed of concave parts are formed in the respective regions located between a data recording region Atz and the preamble pattern region Apz, between the preamble pattern region Apz and the address pattern region Aaz, between the address pattern region Aaz and the burst pattern region Abz, and between the burst pattern region Abz and the next data recording region Atz. In addition, non-servo signal regions Axbz constructed of concave parts are formed in the regions between the respective burst regions Ab1z to Ab4z in the burst pattern region Abz. Here, control signals for tracking servo control are not recorded in the non-servo signal regions Axz, Axbz and the non-servo signal regions Axz, Axbz are almost entirely constructed of the concave parts described above with no convex parts being present. Note that the obliquely shaded areas in FIG. 32 represent the formation regions of the convex parts in the servo pattern 40sz and the data track pattern 40tz. 
Here, various types of concave parts are formed in the data recording regions Atz and the servo pattern regions Asz of the magnetic disk 10z, such as concave parts where the open surface is narrow (for example, concave parts where one of the open length along the direction of rotation of the magnetic disk 10z and the open length along the radial direction of the magnetic disk 10z is short) and concave parts where the open surface is wide (for example, concave parts where both the open length along the direction of rotation and the open length along the radial direction are long). Also, a concave/convex pattern 39z (see FIGS. 33 and 34), where the positional relationship of the convexes and concaves is inversed relative to the concave/convex pattern to be formed on the magnetic disk 10z, is formed on the stamper used to manufacture the magnetic disk 10z. Accordingly, as shown in FIGS. 33 and 34, various types of convex parts 39az such as convex parts with narrow protruding surfaces (the convex parts 39az shown in FIG. 33) and convex parts with wide protruding surfaces (the convex part 39az shown in FIG. 34) are formed on the stamper used to manufacture the magnetic disk 10z. Here, in the conventional manufacturing method, the concave/convex pattern 39z is pressed onto the resin layer with a substantially uniform pressing force across the entire stamper. When doing so, as shown in FIG. 33, at positions where convex parts 39az with comparatively narrow protruding surfaces are formed, the resin material at the positions where the convex parts 39az are pressed in moves smoothly inside the concave parts 39bz of the stamper, which makes it possible to press the convex parts 39az sufficiently deeply into the resin layer. As a result, it is possible to form a concave/convex pattern 41z on the substrate where the thickness T1 of the resin layer between the front ends of the convex parts 39az and the substrate (i.e., at the base parts of concave parts 41bz) is sufficiently thin. It should be noted that in the following description, resin material remaining between base surfaces of concave parts formed in the resin layer and the substrate is referred to as “residue”.
On the other hand, as shown in FIG. 34, at positions where a convex part 39az with a comparatively wide protruding surface is formed, it is difficult for the resin material at the position where the convex part 39az is pressed in to move inside the concave parts 39bz of the stamper, which makes it difficult to press the convex part 39az sufficiently deeply into the resin layer. As a result, residue with the thickness T2 is produced between the front end of the convex part 39az and the substrate (i.e., at the base part of the concave part 41bz), resulting in the depth of the concave part 41bz becoming shallow. Here, when the substrate is etched with the resin layer in which the concave/convex pattern 41z has been formed as a mask, it is necessary to remove the residue on the base surfaces of such concave parts 41bz in the concave/convex pattern 41z by carrying out etching or the like (the removal process mentioned above). Also, as mentioned above, the thickness T1 of the residue at the positions where the convex parts 39az with narrow protruding surfaces are pressed in is considerably thinner than the thickness T2 of the residue at the positions where the convex parts 39az with wide protruding surfaces are pressed in. Accordingly, when the etching process (removing process) is carried out for a sufficiently long time to reliably remove the residue with the thickness T2, the removal of the residue of the thickness T1 is completed before the removal of the residue of the thickness T2 is completed. As a result, at positions where the residue with the thickness T1 is removed (at positions of the concave parts 41bz whose open surfaces are narrow), the inner surfaces of the concave parts 41bz are corroded by the etching gas, irradiation of which continues until the residue with the thickness T2 is removed, which makes the concave parts 41bz excessively wide. This means that with the magnetic disk 10z manufactured according to the conventional manufacturing method, due to the difficulty in forming the concave parts 41bz with the desired width during manufacturing, there is the risk of the concave parts formed in the data recording regions Atz and the servo pattern regions Asz becoming excessively wide, which can result in difficulty in reliably reading magnetic signals.